Continuous facsimile radio recorder



Patented Nov. 18, 1941 UNITED STATES PATENT OFFICE 2,263,333 I CONTINUOUS FACSIMILE RADIO RECORDER William G. H. Finch, Newtown, Conn.

Application April 26, 1938, Serial No. 204,385

Claims.

This invention relates to radio facsimile recording systems and more particularly relates to novel methods of and means for preventing injury or burning of the recording sheet due to signal surges, static or higher than normal signal level.

This application is a continuation-in-part of my copending application Serial No. 203,851, filed April 23, 1938 entitled Continuous facsimile radio recorder.

In a simplification of design of facsimile recorders for widespread home use, efforts have been directed towards providing a fool-proof and automatically operated unit. Vagaries of radio transmission have limited the widespread adoption of continuous dry paper recorders. Electrical scanning of a dry sensitive recording paper generally requires signal intensities of the order of several hundred volts to obtain sharply detailed results, Unexpected surges in the recording signal level presents the hazard of record sheet burning. In order to meet Fire Underwriters requirements, it is essential that the apparatus be fool-proof to the extent of eliminating any possibility of fire by burning of the record sheet. The radio transmission vagaries tending towards the overloading is due to fading where the signal level is unduly increased by surges of static, and other radio disturbances.

Dry type recording paper has a break-down or burning voltage value depending upon its type and the thickness of the surface chemical coating. Thus, a sheet saturated with carbon particles to render it conductive may have a coating of mercurous oxide, titanium oxide or the like, which coating is discolored by the picture signals. A normally prepared sheet is used with a signal voltage level varying to a peak of 200 to 300 volts. A paper with a thicker coating for example, could still render selective marking characteristics up to a value of about 400 volts. By controlling the preparation of the paper, a uniform signal intensity marking characteristic can be insured.

During reception the signal may increase above the three or four hundred volt peak level for which the paper was designed to operate. The markings upon the sheet become excessively dark and the stylus tends to dig in and tear the paper at an excess signal level. Aside from disfiguring the recording and perhaps interfering with the normal continuous operation of the unit, a more important danger arises from the possibility of the paper itself starting to burn at the high signal level.

In accordance with my present invention I contemplate a novel method of limiting the peak signal which is applied directly to the recording sheet. A peak signal limiter is used in conjunction with the amplified facsimile signals before they are applied to the recording sheet. In the preferred embodiment of my present invention, I employ a novel limiter-rectifier circuit which is used in conjunction with carrier frequency currents modulated by the picture signals to rectify the picture signals to their original unidirection character and apply them to the recording sheet through the stylus. I employ a rectifier having a control-grid element located between the anode and cathode thereof, together with means connected to the control-grid for limiting the rectifying action of the tube at the higher values of signal input to prevent the output from becoming excessive.

In accordance with my present invention, I contemplate a rectifier for the facsimile signals, operated in a manner to limit the output signals to a predetermined maximum level to prevent burning or charring of the record sheet. In a preferred arrangement, I connect the control grid element to the anode through a grid resistor. The signal input to the rectifier anode is accordingly applied directly to the control grid through the grid resistance. The fiow of grid current during the positive cycle portion causes a voltage drop through the resistor at the control electrode, reducing its effective potential. The circuit components are designed whereby the control voltage on the grid becomes effective to limit the higher signal values to a predetermined maximum and insure safe recording conditions.

The control-grid resistance may be made variable so that the peak output of the facsimile recorder may be adjusted for different types of recording papers used, in a manner which will be hereinafter described in more detail. In a preferred arrangement, I-employ a double stage rectifier to attain full-wave rectification of the picture modulated carrier signals and to also insure maximum efficiency of rectification of the signals for given conditions. The double stage rectifier comprises a double triode amplifier tube connected as a rectifier with the grid resistance both connected in accordance with the principles of my invention. If a variable grid resistor is used in each section, they are preferably mechanically ganged for a unitary level control setting mechanism.

It is accordingly an object of my present invention to provide a novel method of receiving facsimile signals and applying them to a dry sheet recorder unit.

Another object of my present invention is to provide a novel method of radio facsimile recording to avoid burning of a record sheet.

Still another object of my present invention is to provide novel methods of and means for limiting the peak facsimile signal current applied to a recorder unit.

A further object of my present invention is modulator. nected to the screen-grid electrode I8 of the modulator stage through a coupling condenser unit I9.

the rectification of a modulated carrier signalwith means for operating a control grid in accordance with the signal intensity to prevent.

signal output above the predetermined, level.

These and further objects 'of my invention will become apparent in the following. descrip:

An electronic oscillator 25 is employed to gen erate and supply the 2000 cycle carrier frequency or tone to the modulator stage. Oscillator 25 essentially comprises a double triode electronic tube 26 coupled through transformer element 21 to generate the sustained current oscillations at the required frequency. The output winding 28 of the oscillator has one terminal connected to ground potential and the other terminal contion taken in connection with the drawing in which Figure 1 is a schematic diagram of a. preferred facsimile transmitter incorporating the principles of my present invention. l

' Figure 2 is a schematic diagram of a preferred facsimile recorder operated in accordance with the principles of my present invention.

Referring now to Figure 1, a facsimile scanner unit I is oscillated across an arcuately held record'sheet II by a modified heart-shaped-cam I2. The transmitter scanner schematically here represented may well be the unit such as disclosed in my oe-pending application S. N. 123,770 filed February 3, 1937-, entitled Continuous sheet facsimile system, although other well-known scanning units may be used. The transmitter scanner unit consists essentially of a source of light, generating a beam focused. upon sheet II through element I3. This illustrated facsimile system is similar'to that disclosed in my copen'ding application S. N. 203,222 filed April 21, 1938, entitled Facsimile synchronizing system.

The beam or pencil of light is reflected from the record sheet and focused through tubular lens element I4 upon a photoelectric cell I5 contained within the scanner housing I6. As is well known in thefacsimile art, variations in the shading of the recordingsheet I I along the scanning region correspondingly variably affect the intensity of light impinged upon photoelectric cell I5. The photoel'ectriccell is connected in circuit with the modulator stage for producing electrical signals varying in accordance with the elemental shading of the scanned record sheet.

The modulator stage is preferably a screengrid vacuum tube, the control grid of which is connected to the anodeof the photoelectric cell I 5. A biasing control potentiometer II is employed to control the intensity level of the photoelectric cell variations upon the output of the A carrier frequency current iscon- The combination of electrical actions in the modulator stage generates a picture modulated carrier frequency current which is coupled to the amplifier stage through coupling impedance elements 20. I

The output of the amplifier stage is coupled to a suitable transformer 2| to a schematically indicated transmitter-22 for radio transmission of the signals to a remote receiver stage. It'is to be understood that different communication channels than radio may be employed. The output, impedance of transformer 2I may, for example, be 500 ohms to match the, standard coupling impedance of speech input circuits employed in radio broadcast stages. A'hi'gh 'im pedance level indicator 23 such as a D. B. meter,v

is'corinected across the'outpu't oftransformer 2I" forassisting in the initial adjustments of" the transmitterunit.

nected to two potentiometer element units 30 and 3|. Potentiometer 30 is connected to the coupling condenser I9 for the modulator stage through'cam switch members 32-33. When switch- 32-33 is closed the output of oscillator 25 is directly applied to the screen-grid I8. This connection occurs preferably during one hundred and eighty mechanical degrees of each rotation cycle of control shaft 35. Actuation of cam switch-36 is through cam 31 secured on control shaft 35.

Scanner I0 generates picture signals in its ex cursion from left to right corresponding to a 180 rotation of the control shaft 35. The scanner is returned to its left end position during the remaining 180 rotation of shaft 35 during which time no picture carrier signals are transmitted in the preferred arrangement. Carrier tone switch 32-33 is opened by suitable design of cam 31 at this point. 7

During the return stroke (to the left) of the scanner II], the frequency of the generated current of oscillator 25 is changed to that of the synchronizing frequency. In a preferred example, a 500 cycle synchronizing current is used. The frequency of oscillator 25 may be changed by. shunting a condenser element 38 between grid element 39 and ground. This switching is effected by elements 404I of cam switch 35 and occurs alternately with the removal of carrier current from screen grid I8. By properly proportioning the value of the capacitance of condenser .38, an accurate value of the 500 cycle generationris eifected. It is to be understood that other known expedients for changing the oscillator frequency may be employed for chang- 1 Potentiometer 3 I, being in circuit with the output of oscillator 25, is directly connected to the secondary side of the output. transformer 2I at point 46 through an arrangement for accurately predetermining the' duration of the connection of the synchronizing signal across this point. I provide an arrangement using two switches 42 and 43 connected in series, respectively actuated by earns 44 and 45. Switch 43- is in normally closed circuit relation; switch 44, in normally open circuit-relation. No synchronizing current accordingly flows from potentiometer 3I t0 4$ during the return stroke period of scanner I0 until switch 44 is opened.

Cams 4'4 and 4-5 are designed with short projectin'gni-bs for actuating the associated switches Hand 43. Cam 44 is arranged on shaft 35 to close switch 42 at the predetermined instant the synchronizing signal is to be started for transmission during each cycle. A cam can be depended upon to accurately perform this closing function-. The closure of switch 42 completes the circuit between potentiometer III and point 46. Closure of switch 42 is made to preferably occurwhen scanner II) has practically completed its return stroke.

It will be recalled that during this interval, switch 404| is closed and the output of oscillator 25 is at the synchronizing signal frequency, namely 500 cycles in the present example. The end point of the synchronizing signal transmission is accurately determined by cam 45 which is adjusted to actuate normally-closed switch 43 to open the switch at the proper instant. It will now be evident that an accurate control of the starting and stopping point of the synchronizing signal in each cycle can be accurately prearranged by this means.

The synchronizing signal is ended just before the scanner l approaches its extreme-left position in readiness for its picture scanning excursion. At this time, cam 31 alters the position of cam switch 36, opening blades 40--4l to cause a picture tone of 2000 cycles ,to be generated, and closing plates 32-33 to connect the 2000 cycle tone to the screen grid l8 of the modulator stage. Control shaft 35 is rotated at a predetermined speed such as by a synchronous motor through reduction gearing. In the given example, shaft 35 rotates at 60 revolutions per minute, effecting sixty scanning operations and corresponding synchronizing signal transmission per minute. The picture modulated carrier and synchronizing signals are accordingly alternately transmitted across radio transmitter channel 22 to the remote receiving stage.

Figure 2 is a diagrammatic representation of a preferred system for recording the transmitted facsimile signals. A standard broadcast radio receiver is indicated at 50. The output stage 5| represents the conventional audio-frequency output amplifier stage coupled to loud speaker 52 through step-down transformer 53.

Coupling unit 55 couples the facsimile recorder to the radio receiver. Coupling unit 55 essentially comprises a step-up transformer 56 and a thermionic rectifier unit 51. Primary 56 0f transformer 56 has one terminal connected to ground and the other terminal connected to the radio receiver through a coupling condenser 59. Switch 60 connects transformer 56 to terminal 6| of the radio receiver. A preferred coupling point to the radio receiver is to the anode 62 of the output stage 5'! thereof. This connection insures the maximum voltage signal output from the radio receiver to the facsimile coupling unit 55.

Connecting switch 60 may be manually operated when facsimile reception is to commence. However, it is at present usual to connect the facsimile receiver at the earliest morning hours since the broadcasting of facsimile programs occurs at that time. Accordingly, switch 6| together with the associated switches for starting both the radio receiver 50 and the facsimile system in operation may be time-clock controlled in a manner well known in the art. A second switch 63 is shown mechanically ganged with switch 60 to open the circuit connection to the loud speaker 52 to prevent the facsimile signals from causing a sound disturbance. It is entirely feasible to maintain the loud speaker 52 in circuit to aurally determine the quality and nature of the signals.

In accordance with my present invention, I provide novel means for limiting the peak facsimile recording current as applied to the facsimile recording sheet to prevent overloading, charring or burning of the paper. In the pre ferred embodiment, I employ a novel limiterrectifier circuit arrangement to perform the signal limiting action. Broadly, the rectifier comprises a grid element which is lconductively connected to the signal voltage, which point also preferably connects to the rectifier anode. A grid resistor is inserted between the grid and the signal connection point. The grid functions at a positive potential and draws grid current during signal reception. Grid current flow through the grid resistance causes a drop in potential therein which functions to depress the voltage of the grid.

In accordance with my present invention, the greater the signal voltage impressed upon the anode-grid. of the rectifier, the greater will be the current flow through the grid resistance and the lower will be the grid voltage. The depressed grid voltage serves to limit the rectified current flow through the rectifier. By proper design of the circuit components, the current output of the rectifier can be limited to a substantially predetermined value which lies within the safe limits for operating the recording sheet to prevent burning or charring thereof. Any unexpected increase or surge in signal level is absorbed by this limiter-rectifier circuit. A stepdown power transformer 64 is used to energize the heater element I05 of the rectifier stage 51.

The 2000 cycle modulated carrier picture signal currents received by unit 50 are further amplifled in magnitude by transformer 56 and then rectified at 51 by my novel limiter-rectifier circuit arrangement. The signal energy output is represented between the cathode 65 and ground. Cathode 65 is connected to the scanner 10 through switch blades 61-68 through the oscillating scanner arm 69. Arm 69 is electrically connected to the pivotally mounted stylus point H which bears against record sheet 12. Sheet 12 is secured in position against a metallic platen I3. Platen 13 is electrically connected to ground to complete the picture signal circuit to rectifier 51.

In accordance with my present invention, the maximum value of the signal current impinging upon the record sheet 12 s limited to the predetermined peak value with the advantages and reasons to be hereinafter more fully described. However, before describing the features of my present invention I shall complete the description of the illustrated exemplification of a facsimile system embodying my invention, which system is otherwise similar, for example to that disclosed in my copending application Serial No. 203,222 hereinabove referred to. The diagrammatically illustrated recorder is similar to that described in my copending application Serial No. 156,625, filed July 31, 1937 entitled Facsimile recorder. Record sheet 12 is maintained in position to establish contact with platen l3 and cooperate with the oscillating stylus 1|. Record sheet 12 is a conducting paper which is electrically sensitized in accordance with the picture signals to build up a facsimile reproduction corresponding to that transmitted from scanner Ill. Heart-shaped cam 14, in cooperation with spring 15, produces an oscillating movement of arm 69 about pivot 16.

A non-synchronous motor 11 is preferably employed to drive cam 14 through a reduction gear box 18 and a friction clutch 19. Motor 11 may be an A. C. induction motor, or a motor of the A. C.-D. C. type. An electrical governor is mechanically connected to the rotor of motor 11, and employed to maintain motor 11 at a predetermined speed of rotation. Governor 80 electrically opens and closes the volt power leads to the r" to d ive i a Pro e sea1 akr filter a1 is connected I acrossgovemor an t6 min l imize sparking. Synchronous ing arm 69 is eifec controlled motor chronous actuation o nizing magnet 82 ted by means of the governor r the'clutch 19 symm t scanning movement of oscillate I1 and the start stop syn} is controlled by the short durav Q tion synchronizing signal in a manner to be de-.'

scribed. I have found, that by operating the recorder control shaft 85 at a speed aboutfive per cent greater than that of control shaft 35 of the transmitter, the scanner arm '69 will arrive at its initial left-end position in readiness for its recording stroke; in sufficient time before re.- ception of the synchronizing impulse to start it off again for most operating conditions. Driven plate 86 of clutch 19 contains a projection 81 which abuts armatureBB to stop the rotation of shaft 85 until the synchronizing impulse is received to release armature 38.

Cam switch 90 is maintained in the position illustrated during the returnstroke of oscillator 69. Signal plate 61 is connected to thesynchro' nizing magnet 82 through contact 9|. Normally no signal is impressed upon magnet 82 until a synchronizing impulse is received. I have found that the synchronizing impulse should be of very short duration to initiate the scanning excursion at recorder I0 at the predetermined starting posistant the synchronizing magnet is energized, cam

92 operates cam switch so to the rightto open the synchronizing magnet circuit and close signal blade 61 upon contact 68, therebyimpressing the" I rectified picture signals upon the stylus io r reinsiiretheprevention a, ass-rem with the d cycle tone passingithrough to the rectifier 51 sub-f The 500 cycle synstantially unattenuated. chronizing impulse is rectified at 51 and is applied. ,to the [synchronizing magnet through switch'f8'l 9l as a unidirectional current impulse of short duration, Y

My invention primarily directed to the prevention/oi injury and burning of the. facsimile recording sheet." Signal overloadings due to vagaries of radio transmission, for example, are dangerous in unattended automatic facsimile'recorders particularly those employing dry record paper. A static surge, or undue signal intensity increasedue to fading and the like impresses a voltageintensity beyond that which the paper can primarily stand, and breaks down the paper surface; The fire hazard is naturally more dangerous than destruction ,of portions of the recording; In accordance with my invention I providea signal limiting means which prevents the signal intensity from becoming excessive at the recording paper, and avoids destroying p'orburning of the paper, due to static disturbances and surges I provide an electronic limiter-rectifier arrangement designed and adjusted to prevent the increased power or level input upon the record sheet. e l

accordance with my present invention, I provide means for insuring the maximum output level from therectifier [unit as applied to the recording sheet. Charring or burning signal levels may be predetermined for a given type of recording paper. The signal limiting features of my'invention aredesigned to pass signals up to the predetermined level and prevent signals above that level from harming the record sheet. The application of myinvention resides in automatically controlling the grid bias of a control-grid element in the rectifier, in accordance with the level of the signal voltage applied thereto. The

" grid is normally'operate'd at apositive potential cording on sheet I2 in a m'anner'already described The recording strolte occurs during 1130 mechanical degrees of rotation of control shait at which time the cam switch is released to the illustrated position to repeat"the"cycle ofopera-" tions described.

Means are provi ded I for selectively V the impression of the synchronizing impulsejupon magnet 82 to avoid conflict betweenithe 'synchronizing signal and picture signal at the out- 7 put of the rectifier, Ipr ovide a'filtering means embodied by condenser 93 to by'ipaSS essentially f all the signal frequencies above the synchroniz ing frequency of 500 cycles. The'ZOQ cycle carrier" together with its side vbarrds' otpicture signal modulations' are well above the 500 ycyele value so that the by-passcondenser 93 connected'to one side of the 'secondaryof'the coupling transf former 5B is" usually suiiicient for suchjcasesl Condenser 93 bypasses the higher frequencies since the signal voltage is connected directly to the grid through a gridresistor. However, grid currentproduces a drop across the grid resistor,

reducing the grid potential, in turn limiting the output current between the anode and cathode of the rectifier unitfl min Figure. 2 1 have illustrateda preferred circuit arrangement embodying the features of my present invention. ,The limiter-rectifier unit 56 contains the rectifiertube 51 connected across theouter terminals of the. secondary 96 of coupling transformer 55, Q'Rectifier 51 is of. the double stage type having acommon cathode element '1 E5 energized b'y heater' element I05. Anodes 91 and '99 connect to the outer terminals of secondary winding 96. The gridelectrodes I00 and LIII are also'conne'ctedto theouter terminals of secondary winding 96, but through corresponding grid resistances [Bland I03. At low signal inputs to rectifier 51, the grids I00 and NI are substantially' at the same potential as respective.

from transformer 56 ,toground throughlswitich elements 94-95. More involved filtering means;

actuated bythe camswitch 90 may" be used if more selective signalseparation is'required.

The preferable signal level for operating the re-J j corder I0 is in the order of 100 to 300 yolts. The expedient of the filtering condenser '93 has been t V e I03 used with rectifier output signal voltages of found'to readily reduce the ZOI OcycIe signal level to a value of 15% of the original andthereby anodes 91 and 98; since a negligible drop occurs through grid resistors I02 and I03.

The rectifier stage 51 may well be a power amplifier tube as used inthe output stages of audiofrequency amplifiers and radio sets. The dualtriode typeitubeused as a limiter-rectifier in a preferred showing of my present invention may be a'6A6 type output tube employed in present day radio receivers.f' The grid resistors I02 and" 200 "to 30 0 volts; maybe of the Order or five tions of thereproduction due to overloading or.

megohms each. For a given type of recorder and recording paper, as well as other circuit arrangements, the resistors I02 and I03 may be connected in as fixed resistances. However, should different types of recording paper be desirable, in use, the grid resistors may be made variable, as indicated at I02 and I 03, so that they may be adjusted by the operator in accordance with the type of recording paper used.

Should the resistances I02 and I03 be variable, they are preferably mechanically ganged for unitary control as indicated by dotted connection I04. By proper settings of the control opposite predetermined indications, the peak voltage output of the limiter-rectifier may be readily changed. Thus, if a recording paper having a 200 voltage peak characteristic is employed, a high resistance setting is used to more effectively control the signal limiting feature of the positive grid. Conversely, should a less sensitive recording paper, having a higher burning voltage, say 400 volts, be used, the resistance setting for grid resistors I02 and I03 may be decreased to obtain a less sensitive limiting effect by my invention.

The limiter feature herein described and illustrated is adaptable for different types of facsimile circuits and arrangement, and different types of recording paper. The circuit may be a double triode as shown, or only a single section. It is to be understood that modifications may be exercised in carrying out the principles of my invention. By-pass condensers may be shunted across the respective grid resistors I 02 and I03 to smooth the unidirectional voltage thereacross. Furthermore, my present current limiting invention may be combined with the limiting feature disclosed in my co-pending application Serial No. 203,851, filed April 23, 1938, hereinabove referred to.

By operating the cathode 65 so that its output would be at saturation level for maximum signal output, with the control grid resistance circuits illustrated at I02-I03, together with control grids 9198, a doubly effective output-currentlimiting device is had. Thus, by properly designing the surface and size of cathode 65 as well as the operating voltage of heater I05, in a manner disclosed in my co-pending application, the electron emission from the cathode 65 could be prevented from becoming excessive in any event. The combination of the control grid feature and emission-limiting feature effect a foolproof positive control for preventing burning and charring of the record sheet. Reference is herein made to the operating conditions and features disclosed in the co-pencling application regarding the saturation cathode feature operation of the rectifier unit, as well as the compensator circuits which may be employed to obtain an over-all linear marking at the recorder. The compensator unit disclosed in my co-pending application Serial No. 203,851, herein referred to may be advantageously employed with any type of current limiting circuit including the one forming the basis of my present application and insuring an over-all marking characteristic.

Having disclosed a preferred arrangement and embodiment for carrying out the principles of my present invention, it is to be understood that it is amenable to modifications by those skilled in the art without departing from the broader spirit and scope thereof as defined in the appended claims.

What I claim is:

1. A facsimile recording system comprising means for receiving and amplifying facsimile signals; means for rectifying said signals comprising a rectifier tube including an anode, cathode and control-grid electrode, and means including circuit connections to said control electrode for limiting the magnitude of the signal output from said rectifying means comprising an impedance element connected between said control-grid and anode; and a recorder unit electrically operable on a paper sheet, connected to the output of said rectifier.

2. A facsimile recording system comprising means for receiving and amplifying facsimile signals including an output transformer; means for rectifying said signals comprising a rectifier tube including an anode, cathode and control-grid electrode, and means including circuit connections to said control electrode for limiting the magnitude of the signal output from said rectifying means comprising a resistor connected between said control-grid and anode, said anode being connected to the output of said transformer; and a recorder unit electrically operable on a paper sheet connected to said rectifier cathode, said resistor being adjusted to limit the signal applied to said paper to a predetermined maximum value whereby burning of the paper is prevented.

3. A facsimile recording system comprising means for receiving and amplifying facsimile signals; means for rectifying said signals comprising a rectifier tube including an anode, cathode and control-grid electrode, and means including circuit connections to said control electrode for limiting the magnitude of the signal output from said rectifying means comprising an impedance element connected between said control-grid and anode; and a recorder unit electrically operable on a paper sheet, connected to the output of said rectifier; said cathode having an emission characteristic which predetermines the peak output of said rectifier to said paper.

4. A facsimile recording system comprising means for receiving and amplifying facsimile signals including an output transformer; means for rectifying said signals comprising a rectifier tube including an anode, heatable emission element and control-grid electrode, and means including circuit connections to said control electrode for limiting the magnitude of the signal output from said rectifying means comprising a resistor connected between said control-grid and anode, said anode being connected to the output of said transformer; and a recorder unit electrically operable on a paper sheet connected to said heatable emission element, said resistor being adjusted to limit the signal applied to said paper to a predetermined maximum value whereby buming of the paper is prevented; said heatable emission element having an emission characteristic which predetermines the peak output of said rectifier to said paper.

5. In a signalling system, means for receiving and amplifying signals, an output transformer connected to said means, means connected to the secondary of said transformer for rectifying said signals comprising a rectifier tube including an anode control electrode and a cathode, and means including circuit connections to said control electrode for limiting the magnitude of the signal output from said rectifying means comprising a resistor connected between said control electrode and anode, said anode being connected to the output of said transformer.

WILLIAM G. H. FINCH. 

